Generally, a secondary battery includes a battery case, an electrolyte and an electrode assembly accommodated in the battery case.
The electrode assembly has a structure in which cathodes, separators and anodes are stacked in turns. The cathode and the anode of the electrode assembly respectively include current collectors made of aluminum foil (Al-foil) and copper foil (Cu-foil). The cathode current collector and the anode current collector are respectively coated with a cathode active material and an anode active material, and a region not coated with an active material, namely, a non-coated region, is connected to an electrode tab.
In order to obtain uniform charge/discharge characteristics of the secondary battery, the current collectors should be precisely coated with a cathode active material layer and an anode active material. For this, a slot-coating process is generally performed.
FIG. 1 shows a general slot die for performing a slot-coating process. As shown in FIG. 1, the slot die includes a feed unit 10 for introducing an active material coating solution, a chamber 11 for accommodating a coating material supplied by the feed unit 10, a slit 12 communicating with the chamber 11 to discharge the coating material, and die lips 13 which form the slit 12.
The coating solution flows into the chamber 11 through the feed unit 10, fills the chamber 11 and then discharges out through the slit 12.
The coating solution discharging through the slit 12 is successively coated onto a substrate. The active material coated onto the substrate has a coating width which is determined by the width of the slit 12.
A slot-coating apparatus is disclosed in Korean Unexamined Patent Publication No. 2011-0098578 and Korean Unexamined Patent Publication No. 2004-0084013.
Korean Unexamined Patent Publication No. 2011-0098578 discloses a method for designing an interior of a slot-coating die for successive and uniform coating of an electrode-coating catalyst slurry, which includes selecting a fluid model for analyzing flow characteristics of a fluid in a die from rheologic property information of a catalyst slurry fluid, determining a chamber shape in the slot-coating die in consideration of the rheologic characteristics of the catalyst slurry fluid, and determining a process condition for forming a catalyst layer of a uniform thickness. In addition, Korean Unexamined Patent Publication No. 2011-0098578 discloses a semi-cylindrical chamber having a center portion of a convex hanger shape in which a catalyst layer of a uniform thickness is formed by determining a width of the convex region.
Korean Unexamined Patent Publication No. 2004-0084013 discloses a precise coating apparatus and method for discharging a coating solution in an optimal condition by installing exchangeable lips of various shapes. Korean Unexamined Patent Publication No. 2004-0084013 discloses a precise coating apparatus including a fixed block, an upstream die and a downstream die respectively installed at an upstream die block and a downstream die block mounted to the fixed block, an upstream lip and a downstream lip exchangeably fixed to the upstream die and the downstream die, respectively, and a coating solution supply manifold.
When it is needed to change a coating width, in a general slot die, a design size of a shim plate which determines widths of a slit and an inner space of the chamber is adjusted to implement various coating widths. FIGS. 2a and 2b schematically show an example in which a size of a shim plate 14 inserted into a die body 15 is controlled to change a width of the inner space of the chamber 11, thereby adjusting the coating width. However, in this method, if the size of the shim plate 14 is adjusted to change over 10% of the coating width, the slurry cannot discharge out of a dead zone in the chamber 14, namely at both edge portions of the chamber 14, but stays thereat, and the accumulation of slurry and vortex may deteriorate the coating property and the uniform coating thickness.
In addition, in the general slot die, whenever the coating width is adjusted, the die body must also be newly designed and fabricated, which consumes much time and costs.